The present invention generally relates to cementing, and, more specifically, to methods for cementing within confined locales and cement compositions adapted for the same.
Cement is a binding material that may be used in a number of different applications to form structures having high mechanical strength. Hydraulic cements, which cure or set through chemical reactions initiated by contact of a dry cement with water, are very commonly used in structural applications. The terms “cure,” “set,” and variants thereof will be used interchangeably herein. Typically, a dry cement and water are combined to form a cement slurry, which may then be directly formed into a desired shape or pumped to a desired location before being formed into a desired shape and set into a hardened state.
Cements are commonly used in the drilling industry for the completion and repair of oil and gas wells. For example, cements can be used for securing the drilling pipe (i.e., the drill string) in a wellbore. In primary cementing operations, a cement slurry may be introduced to the annular space between the drilling pipe and the exterior walls of the wellbore, with the cement slurry thereafter being allowed to set into a hardened state. The cement slurry may be introduced directly to the annular space or indirectly by forcing it down the drilling pipe and back up the annular space. Not only does the hardened cement physically stabilize the wellbore pipe, but it also inhibits the unwanted migration of fluids between different zones of the wellbore via the annular space. In addition to filling the annular space of a wellbore, cements can be used to plug lost circulation and outflow zones in a wellbore, repair cracks and holes in a previously cemented wellbore, as well as to accomplish other well remediation operations. Further, cements can be used to permanently close a wellbore that is no longer in production and is being abandoned.
The rate of curing of a cement to form a hardened state may ultimately determine the success of a downhole cementing operation. More generally, the rate of curing may determine whether a particular cement can be effectively used in a given application. When introduced into a subterranean formation, the uncured cement may be exposed to high temperatures that can lead to undesirably rapid curing in some cases. When curing occurs more rapidly than desired, a set retarder may be added to the uncured cement in order to delay its cure. On the other hand, it may sometimes be desirable to accelerate the curing of a cement slurry by adding a setting accelerator. For example, when a subterranean formation is very porous, it may be desirable for the cement slurry to be set as soon as feasible after being delivered to a desired location in order to limit cement leak off into the formation.
When cementing the annular space of normal oil and gas wellbores, correct placement of an uncured cement therein is ordinarily not a particularly pressing concern, since the cement's curing time can normally be modulated as needed through use of set retarders or set accelerators. In addition, the relatively large annular spaces of these types of wellbores do not make it particularly difficult to introduce cement thereto.
In contrast to oil and gas wellbores, boreholes utilized for mineral exploration beneath the earth's surface are generally much smaller in size. For example, when conducting wireline operations in mineral exploration boreholes, the annular space within the borehole may be only a few millimeters wide. Mineral exploration may involve mapping a subterranean space for a mineral deposit. Mineral deposits may include both precious and non-precious metals, non-metals, and any compound thereof (i.e., metal ores).
Like typical oil and gas wellbores, it can sometimes be desirable to perform cementing operations in wellbores used for mineral exploration. However, the very narrow annular spaces in such wellbores may make effective delivery and proper placement of the uncured cement very difficult. Specifically, under-pumping or, more likely, over-pumping of the uncured cement can commonly occur when cementing mineral exploration wellbores. Under-pumping or over-pumping can result in the delivery of the cement to a location in the wellbore where cementing is not desired or, even worse, to a location that damages the wellbore. Further, the types of cementing equipment typically used for cementing oil and gas wellbores may not be capable of being adapted for cementing mineral exploration wellbores without significantly modifying the uncured cement. Significant modifications to the cement may result in unintended consequences, such as changing its curing rate or mechanical strength when hardened.
In addition to the foregoing, proper retention of water in the uncured cement may be difficult to maintain for effective cement curing and delivery to take place. Especially if the wellbore is highly porous, loss of water from the uncured cement can prevent it from curing properly, even if delivered to a desired location in a wellbore. Poor curing can render the cement ineffective for its intended purpose in the wellbore. Poor cement jobs can lead to a reduced ability to core a mineral deposit, either in having to collect a reduced core size or not having the ability to core certain sections of the mineral deposit. Having a reduced ability to core may, in turn, affect the valuation of a project. Another problem caused by water loss from the uncured cement can be loss of fluidity of the cement slurry, which may again result in the cement being delivered to a location different than that intended.